The rotor of a direct drive generator for wind turbines has a diameter of more than 4 m (meters), dependent on the generator power and torque. The generator rotor of the Siemens Wind Power direct drive turbines for example has an outer diameter of 4.2 m for the SWP 3.0-101 and an outer diameter of 6.5 m for the SWP 6.0-154 and a length of around 2.5 m.
The rotor consists of a front plate and a rotor ring with a certain yoke height. The complete rotor is also called rotor housing because in the Siemens Wind Power direct drive turbines the rotor is located outside of the stator and acts therefore like a house. The rotor outer surface is directly in contact with the ambient air.
The rotor housing of all the direct drive Siemens Wind Power generators is a single welded and machined steel component. The rotor housing consists of two large cones, which are welded together with a forged steel ring and a rolled steel yoke. This rotor housing is then machined in a large CNC machine. That process is taking a very long time and requires very fine tolerances. Therefore, the rotor housing is fixed in a large tool, while it is machined in order to keep the roundness of the yoke. This makes it expensive, especially for bigger diameter like on the 6.0 MW generators, and the number of suppliers who can deliver such a steel component is limited. Furthermore there may be occur issues in the supply chain with this manufacturing process due to the total process time.
Therefore, the supply chain is limited to few suppliers who are capable of handling parts on the size of the rotor housing described before, and all suppliers have in common that they need to invest in new equipment and machinery in order to be able to produce such rotor housings. For example, a milling machine capable of machining a rotor housing for a 6.0 MW (megawatt) turbine may costs some millions of Euro, with a capacity of maybe roughly below one hundred units per year. Suppliers going for such an investment may require a longterm commitment from a wind turbine provider, which may limit the possibilities for changing design and supply setup of a wind turbine. Moreover, such a wind turbine provider may also be limited to only use suppliers close to the wind turbine provider production facilities, due to the high cost and limitations related to transportation of the rotor housing.
EP 2578872 discloses a generator, particularly for a wind turbine, having a rotor and a stator, whereby the rotor comprises at least one disc-like shaped rotor end plate supporting the rotor and the stator comprises at least one disc-like shaped stator end plate supporting the stator, wherein the at least one rotor end plate and/or the at least one stator end plate at least partially extends tilted relative to the center axis of the generator. An aspect is to provide a generator with improved mechanical stability.
The problem is to produce such a rotor with the given dimensions in a serial production in a more cost and time efficient way. Only a couple of suppliers worldwide are able to produce such large, single welded and machined steel components with the needed precision regarding component tolerances in the field of wind turbines. In addition the rotor needs to be transported from the supplier to the wind turbine provider, which is costly for the given dimensions.
WO2011031165 discloses a wind turbine comprising an integrated and segmented permanent magnet generator without the traditional end covers with bearings and shaft making it possible to manufacture and transport generator segments consisting of both generator stator and rotor segments as ready to assemble and already positioned to each other as easy transportable elements to an assembly place where they are integrated with the generator rotor and bearing system of the wind turbine to form a complete wind turbine driveline with blades, hub, bearing unit and an integrated permanent magnet generator with concentric air gap between generator stator and rotor. The generator has an external stator and an internal rotor.
EP2590301 discloses generator assembly, particularly for a wind turbine, having a generator with a segmented rotor comprising a number of rotor segments and a segmented stator comprising a number of stator segments, wherein radially oppositely disposed rotor and stator segments each comprise at least one reception portion for detachably receiving at least one connection means, with the connection means being adapted to detachably mechanically connect respective radially oppositely disposed rotor and stator segments to build stator-rotor-segment-units.
Hence, there may be a need to produce a direct drive generator with an external rotor in an easier and cost reducing manner.